Mineral Oil is the cheapest and easiest way to finish a butcher block. Easy and Cheap! and DIY! You don't want to leave a butcher block unfinished because the wood can easily become damaged/stained/moldy, so applying mineral oil periodically keeps it nice and useable. You need to re-finish the block from time to time as the mineral oil is not a permanent finish (there are no permanent finishes for butcher blocks). There are other, more expensive, ways to finish butcher blocks, but many products used for this purpose also contain mineral oil and are way more expensive. (Note: do NOT use Mineral Spirits, which is paint thinner).
Hardest part? Finding Mineral Oil. It's primarily sold as a laxative, so look for it at your local pharmacy/drug store (I got mine at Walgreens). I wasted a lot of times going through kitchen sections and looking at other types of wood finishes at other stores before realizing it's a laxative and sold in a completely different store than the hardware store I was at.
How to use: Take unfinished butcher block (or butcher block in need of re-finishing) and pour Mineral Oil on top. Use a clean rag to spread the Mineral Oil around, and let it sink into the wood. When the oil stops sinking into the wood, you have finished! Wipe off the excess. The oil should settle pretty quickly, so you can use the butcher block shortly thereafter.
Below: Before and After photos of my newly finished butcher block.